In fossil fuel motors, especially internal combustion engines, when a direct injection system is used, the fuel for combustion is injected directly into the combustion chamber. An injection system frequently used for this purpose is common-rail injection. In common rail injection systems a high-pressure pump pressurizes the fuel to a high level. The pressurized fuel fills a system of tubes or one or more pressure lines, which is or are constantly under pressure while the engine is in operation. The fuel is injected into the combustion chamber by actuating electrically-operated injection valves connected to the pressure line. The injection time and the injection amount are controlled by the engine electronics which issues the corresponding injection signals to the injection valves. Such common-rail injection systems can be used for gasoline engines and especially for diesel engines for example.
The size of the quantity injected into a cylinder in each case influences factors such as the combustion cycle, the emissions, the torque produced and the noise produced by the engine.
The injection quantity is determined by factors such as the valve opening time and the pressure in the pressure line. To actually enable the desired injection volume for an injection to be injected into a cylinder with high accuracy, it is thus necessary to be able to adjust the pressure in the common-rail injection system, i.e. in the pressure line or lines, as precisely as possible.
Closed-loop pressure control is thus provided in common-rail-injection systems, by means of which the pressure in the injection system is controlled to a setpoint pressure value, which as a rule depends on operating parameters of the internal combustion engine. Basically simple closed-loop control can be used for this purpose. The disadvantage of this however is that, if there is a rapid change in the setpoint pressure value, significant time is needed for the closed-loop control system to adjust the current pressure value to the setpoint pressure value.
It is thus conceivable to use a pilot control for closed-loop control. This is used, depending on the values of the operating parameters, to adjust the common-rail injection system or devices within it by means of which its pressure can be influenced, so that, even on the basis of the pilot control, the setpoint pressure value is obtained as well as possible and precision control around the operating point predetermined in this way only then has to compensate for small control deviations. In this way the actual pressure value can be rapidly adjusted to the setpoint pressure value even if there are rapid changes to the latter.
Engine maps can be used for pilot control for example, which, depending on an operating parameter in each case, represent a contribution for a value to be set for the devices. The engine maps are determined in such cases for a predetermined type of common-rail injection system, for example using a standard system with modules used as standard or by forming an average from a number of common-rail injection systems of the same type used for determination.
Manufacturing tolerances and wear and tear in injection systems however lead to the accuracy of the engine map for an individually determined common-rail injection system not being as great as desired or to the accuracy reducing over time. In particular the characteristics of an individually predetermined common-rail injection system can deviate from the values assumed or determined on creation of the engine map, for example of the standard system, so that the engine maps do not allow precise pilot control. The result is that it can take an inordinately long time for the closed-loop control to correct the actual pressure to the setpoint pressure, since after a change in the operating parameters defining the operating point of the internal combustion engine and thereby the setpoint value as well as the pilot control, a relatively great control deviation can occur.